I have been using Calibre successfully on a Snow Leopard Mac and am very impressed by the work that's gone into it.

Yesterday I downloaded Calibre for my wife's slightly older iMac still running Tiger. It installs from the dmg okay, but when double-clicked, the bouncing icon appears on the dock, then just as quickly closes and disappears from the dock. No crash as such, it just won't stay launched. The Force Quit display window shows no Calibre process running.